There are known various methods and devices, which have been devised for measuring of internal pressure within a pneumatically inflated tire of a vehicle. Those means can be divided into various groups according to the principle of their operation:
The first group includes monitoring systems mounted on a vehicle wheel and capable to detect and indicate the pressure within a tire when the vehicle moves and the wheel spins. Those systems usually comprise non-contact sensors attached to the wheel and capable to activate an alarm to alert a driver that the pressure in one of his tires is abnormally low. An example of such a system is a tire pressure sensing system disclosed in U.S. Pat. No. 4,330,774.
The second group refers to so-called pressure gauges operating without air connection with the tire and suitable for measuring of internal pressure of a non-moving wheel.
In U.S. Pat. No. 4,615,211 is disclosed a pressure gauge for tires and other elastic vessels which measures the pressure by sensing deformation of a wall of the vessel without establishing fluid communication with the interior of the vessel. The deformation within the wall of the vessel is induced by engagement of the wall by a deformable element. The engagement may be achieved by moving either the gauge or the vessel: the gauge could be swung to strike a tire or the tire could be driven onto the gauge. The pressure gauge comprises a deformable element, a rigid guard element surrounding the deformable element and means for sensing the deformation of the deformable element.
Still further group refers to gauges mounted on a wheel so as to be in fluid communication with the interior of a tire. Those devices operate when the wheel is not moving. An example of such a system is an electronic tire gauge disclosed in U.S. Pat. No. 5,394,343. This system comprises valve stem, fitting means for receiving the valve stem of a tire and pressure transducer means coupled to the stem fitting means and capable to generate a pressure signal representing the pressure sensed. The measured pressure is transmitted outside as a radio signal, received by an external receiver, which transfers the radio signal into electric signal, corresponding to the measured pressure and displays its value. The disadvantage of this system is associated with the necessity to provide transducer means with a dedicated source of energy, which renders the whole gauge complicate, expensive and less reliable. Furthermore, such system creates noise, since the transducer means broadcasts the radio signal permanently.
Another example of pressure measuring device, based on the above principle of transmitting pressure signal to an external receiver is described in U.S. Pat. No. 5,335,541. In this patent is disclosed pressure measuring device, which consists of discrete pressure sensing member, located on a tire and of a handheld portable member, which is brought into proximity with the pressure sensing member, when it is required to measure the pressure. The portable handheld member is provided with an electrical power source, power signal generator means and power signal transmission means, which is adapted to transmit the power signal to the discrete pressure sensing member in order to energize it. The handheld member is provided also with appropriate pressure signal receiver means for receiving pressure signal output from the pressure sensing member and with pressure signal processor means and display means for displaying an inflation pressure value generated by the signal processor means. The pressure sensing member is provided with pressure cell means, power signal receiver means, with power signal conditioning means, signal generator means and transmission means.
In this pressure-measuring device power is transmitted between the handheld apparatus and the pressure sensing apparatus by electromagnetic induction and therefore a power-transmitting coil is provided in the handheld member, which should be put over corresponding power-receiving coil, mounted in the pressure sensing member. The handheld member is configured as a cup attachable to a cylindrical portion of the pressure sensing member, which projects from the wheel hub. Since the power-receiving coil of the pressure sensing member resides in the cylindrical portion, it should always be brought in alignment with the cup to enable sliding fit with the power-transmitting coil of the handheld member.
Accordingly an operator should always make sure that the handheld member is properly put on the pressure sensing member. This requirement renders the whole apparatus inconvenient in exploitation, especially when it is used in conditions of bad weather, limited visibility or in darkness.
Besides such pressure sensing member cannot be attached to any standard valve of a vehicle tire without introducing changes in the standard wheel design.
Furthermore, exploitation of the apparatus requires adjusting the existing hub to make possible mounting thereon of the pressure sensing member.
The above-described pressure sensing member has relatively complicated construction, which includes pressure cell, tuning capacitor, pressure cell and three separate printed circuit boards. This renders the whole device relative expensive and less reliable.
It should be born in mind, however that this apparatus has been devised primarily for measuring pressure in large tires, namely aircraft undercarriage wheel tires, where the above disadvantages presumably are less appreciable. However, in the applications, which require measuring inflation pressure in a tire of a relatively small or a medium-sized vehicle, e.g. private car, autobus, truck, tractor etc., this apparatus in fact would be inferior and even disadvantageous in comparison with traditional, commonly used pneumatic pressure gauge.
Thus, despite existence of various devices, which have been devised for measuring inflation pressure, there still exists a need in a new and improved device, which combines advantages of the known pressure-measuring devices, but is free of their drawbacks.